steer-by-wire: implications for vehicle handling and safety
DESCRIPTION
What is by-wire? Replace mechanical and hydraulic control mechanisms with an electronic system. Technology first appeared in aviation: NASA’s digital fly-by-wire aircraft (1972). Today many civil and most military aircraft rely on fly-by-wire. Revolutionized aircraft design due to improved performance and safety over conventional flight control systems. Source: USAF Source: Boeing Source: NASA Source: NASATRANSCRIPT
Steer-by-Wire: Implications for Vehicle Handling and Safety What
is by-wire? Replace mechanical and hydraulic control mechanisms
with anelectronic system. Technology first appeared in aviation:
NASAs digital fly-by-wireaircraft (1972). Today many civil and most
military aircraft rely on fly-by-wire. Revolutionized aircraft
design due to improved performance andsafety over conventional
flight control systems. Source: USAF Source: Boeing Source: NASA
Source: NASA Automotive applications for by-wire
By-wire technology lateradapted to automobiles:throttle-by-wire and
brake-by- wire. Steer-by-wire poses a moresignificant leap
fromconventional automotivesystems and is still severalyears away.
Just as fly-by-wire did toaircraft, steer-by-wire promisesto
significantly improve vehiclehandling and driving safety. Source:
Motorola Outline Introduction Car as a dynamic system Tire
properties
steering system vehicle control estimation conclusion Outline
Introduction Car as a dynamic system Tire properties Basic handling
characteristics and stability Vehicle control Estimation Conclusion
and future work introduction steering system vehicle control
estimation conclusion Why do accidents occur? 42% of fatal crashes
result fromloss of control (EuropeanAccident Causation
Survey,2001). In most conditions, a vehicleunder proper control is
very safe. However, every vehicle hasthresholds beyond which
controlbecomes extremely difficult. The car as a dynamic
system
introduction steering system vehicle control estimation conclusion
The car as a dynamic system Assume constantlongitudinal speed, V,
soonly lateral forces. Yaw rate, r, and sideslipangle, b,
completelydescribe vehicle motionin plane. Force and massbalance:
Linear and nonlinear tire characteristics
introduction steering system vehicle control estimation conclusion
Linear and nonlinear tire characteristics Lateral forces
aregenerated by tire slip. Ca is called tire corneringstiffness. At
large slip angles, lateralforce approaches frictionlimits. Relation
to slip anglebecomes nonlinear nearthis limit. Linearized vehicle
model
introduction steering system vehicle control estimation conclusion
Linearized vehicle model Equations of motion: Valid even when
tiresoperating in nonlinear regionby approximating nonlineareffects
of the tire curve. Handling characteristics determined by physical
properties
introduction steering system vehicle control estimation conclusion
Handling characteristics determined by physical properties Define
understeer gradient: A car can have one of three characteristics:
understeering neutral steering oversteering - + Kus less responsive
more responsive Understeering Negative real roots at low
speed.
introduction steering system vehicle control estimation conclusion
Understeering Negative real roots at lowspeed. As speed increases,
polesmove off real axis. Understeering vehicle is alwaysstable, but
yaw becomesoscillatory at higher speed. Oversteering Negative real
roots at low speed.
introduction steering system vehicle control estimation conclusion
Oversteering Negative real roots at low speed. As speed increases,
one polemoves into right half plane. At higher speed,
oversteeringvehicle becomes unstable! Analogy to unstable aircraft:
themore oversteering a vehicle is,the more responsive it will be.
introduction steering system vehicle control estimation conclusion
Neutral steering Single negative real root dueto pole zero
cancellation. Always stable with first orderresponse. This is the
ideal handlingcase. Not practical to design thisway: small changes
inoperating conditions(passengers or cargo, tirewear) can make
itoversteering. Real world example: 15 passenger van
rollovers
introduction steering system vehicle control estimation conclusion
Real world example: 15 passenger van rollovers Full load of
passengers shifts weight distribution rearward. Vehicle becomes
oversteering, unstable while still in linear handlingregion. Full
load also raised center of gravity height, contributing to
rollover. How are vehicles designed?
introduction steering system estimation vehicle control conclusion
How are vehicles designed? Most vehicles designed to be
understeering (by tire selection,weight distribution, suspension
kinematics). Provides safety margin. Compromises responsiveness.
What if we could arbitrarily change handling characteristics? Dont
need such a wide safety margin. Can make vehicle responsive without
crossing over toinstability. Can in fact do this with combination
of steer-by-wire and statefeedback! introduction steering system
vehicle control estimation conclusion Prior art Active steering has
been demonstrated using yaw rate andlateral acceleration feedback
(Ackermann et al. 1999, Segawa etal. 2000). Yaw rate alone not
always enough (vehicle can have safe yawrate but be skidding
sideways). Many have proposed sideslip feedback for active steering
intheory (Higuchi et al. 1992, Nagai et al. 1996, Lee 1997, Ono
etal. 1998). Electronic stability control uses sideslip rate
feedback tointervene with braking when vehicle near the limits (van
Zanten2002). No published results for smooth, continuous handling
controlduring normal driving. Research contributions
introduction steering system vehicle control estimation conclusion
Research contributions An approach for precise by-wire steering
control taking into accountsteering system dynamics and tire
forces. Techniques apply to steer-by-wire design in general. The
application of active steering capability and full state feedbackto
virtually and fundamentally modify a vehicles
handlingcharacteristics. Never done before due to difficulty in
obtaining accurate sideslipmeasurement, and There just arent that
many steer-by-wire cars around. The development and implementation
of a vehicle sideslip observerbased on steering forces.
Two-observer structure combines steering system and vehicle
dynamicsthe way they are naturally linked. Solve the problem of
sideslip estimation. Outline Steering system: precise steering
control
introduction steering system estimation vehicle control conclusion
Outline Steering system: precise steering control Conversion to
steer-by-wire System identification Steering control design Vehicle
control Estimation Conclusion and future work Conventional steering
system
introduction steering system estimation vehicle control conclusion
Conventional steering system Conversion to steer-by-wire
introduction steering system estimation vehicle control conclusion
Conversion to steer-by-wire Steer-by-wire actuator
introduction steering system estimation vehicle control conclusion
Steer-by-wire actuator Steer-by-wire sensors
introduction steering system estimation vehicle control conclusion
Steer-by-wire sensors Force feedback system introduction steering
system estimation
vehicle control conclusion Force feedback system System
identification
introduction steering system estimation vehicle control conclusion
System identification Open loop transfer function. Closed loop
transfer function. Closed loop experimental response
introduction steering system estimation vehicle control conclusion
Closed loop experimental response test_11_13_pb Bode plot fitted to
ETFE
introduction steering system estimation vehicle control conclusion
Bode plot fitted to ETFE test_11_13_pb System identification
introduction steering system estimation vehicle control conclusion
System identification Bode plot confirms system to be second order.
Obtain natural frequency and damping ratio from Bode plot. Solve
for moment of inertia and damping constant. Adjust for Coulomb
friction. Identified response with friction
introduction steering system estimation vehicle control conclusion
Identified response with friction Not perfect, but we have
feedback. test_11_13_pb What do you need in a controller?
introduction steering system estimation vehicle control conclusion
What do you need in a controller? Actual steer angle shouldtrack
commanded angle withminimal error. Initially consider no tire-to-
ground contact. actuator torque commanded angle (at handwheel)
actual angle (at pinion) effective moment of inertia effective
damping Feedback control only introduction steering system
estimation
vehicle control conclusion Feedback control only test_12_3_b0_j0
Feedback with feedforward compensation
introduction steering system estimation vehicle control conclusion
Feedback with feedforward compensation test_12_3_b0_j0 Feedforward
and friction compensation
introduction steering system estimation vehicle control conclusion
Feedforward and friction compensation test_12_3_b0_j0 Vehicle on
ground (Same controller as before) introduction
steering system estimation vehicle control conclusion Vehicle on
ground (Same controller as before) test_12_3_b0_j0 Aligning moment
due to mechanical trail
introduction steering system estimation vehicle control conclusion
Aligning moment due to mechanical trail Part of aligning moment
from the wheel caster angle. Offset between intersection of
steering axis with ground andcenter of tire contact patch. Lateral
force acting on contact patch generates moment aboutsteer axis
(against direction of steering). Aligning moment due to pneumatic
trail
introduction steering system estimation vehicle control conclusion
Aligning moment due to pneumatic trail Other part from tire
deformation during cornering. Point of application of resultant
force occurs behind center ofcontact patch. Pneumatic trail also
contributes to moment about steer axis(usually against direction of
steering). Controller with aligning moment correction
introduction steering system estimation vehicle control conclusion
Controller with aligning moment correction test_12_3_b0_j0 From
steering to vehicle control
introduction steering system estimation vehicle control conclusion
From steering to vehicle control Disturbance force acting on
steering system causes trackingerror. Simply increasing feedback
gains may result in instability. Since we have an idea where the
disturbance comes from, wecan cancel it out. We now have precise
active steering control via steer-by-wiresystemwhat can we do with
it? Outline Steering system: precise steering control
introduction steering system estimation vehicle control conclusion
Outline Steering system: precise steering control Conversion to
steer-by-wire System identification Steering control design Vehicle
control: infinitely variable handling characteristics Handling
modification Experimental results Estimation Conclusion and future
work Active steering concept
introduction steering system estimation vehicle control conclusion
Active steering concept One of the main benefits of steer-by-wire
over conventionalsteering mechanisms is active steering capability.
For a conventional steering system, road wheel angle has adirect
correspondence to driver command at the steering wheel. driver
conventional steering system vehicle environment steer angle
vehicle states command angle Active steering concept
introduction steering system estimation vehicle control conclusion
Active steering concept For an active steering system, actual steer
angle can be differentfrom driver command angle to either alter
drivers perception ofvehicle handling or to maintain control during
extrememaneuvers. driver vehicle environment command angle vehicle
states controller active system steer angle Physically motivated
handling modification
introduction steering system estimation vehicle control conclusion
Physically motivated handling modification Automotive racing
example: driver makes pit stop to changetires. Virtual tire change:
effectively alter front cornering stiffnessthrough feedback. Full
state feedback control law: steer angle is linear combinationof
states and driver command angle. Obtain sideslip from GPS/INS
system (Ryus PhD work). Physically motivated handling
modification
introduction steering system estimation vehicle control conclusion
Physically motivated handling modification Define new cornering
stiffness as: Choose feedback gains as: Vehicle state equation is
now: Experimental testing at Moffett Field
introduction steering system estimation vehicle control conclusion
Experimental testing at Moffett Field Unmodified handling: model
vs. experiment
introduction steering system estimation vehicle control conclusion
Unmodified handling: model vs. experiment Confirms model parameters
match vehicle parameters. mo_1_3_eta0_d Experiment: normal vs.
reduced front cornering stiffness
introduction steering system estimation vehicle control conclusion
Experiment: normal vs. reduced front cornering stiffness Difference
between normal and reduced cornering stiffness. mo_1_3_a05u_b
Reduced front cornering stiffness: model vs. experiment
introduction steering system estimation vehicle control conclusion
Reduced front cornering stiffness: model vs. experiment Understeer
characteristic in yaw exactly as predicted. mo_1_3_a05u_b
Unmodified handling: model vs. experiment
introduction steering system estimation vehicle control conclusion
Unmodified handling: model vs. experiment Verifies sideslip
estimation is working. mo_1_3_eta0_d Reduced front cornering
stiffness: model vs. experiment
introduction steering system estimation vehicle control conclusion
Reduced front cornering stiffness: model vs. experiment Understeer
characteristic in sideslip as predicted. mo_1_3_a05u_b Modified
handling: unloaded vs. rear weight bias
introduction steering system estimation vehicle control conclusion
Modified handling: unloaded vs. rear weight bias Reducing front
cornering stiffness returns vehicle to unloaded characteristic.
mo_2_3_eta02u_w_b From control to estimation
introduction steering system estimation vehicle control conclusion
From control to estimation We need accurate, clean feedback of
sideslip angle to smoothlymodify a vehicles handling
characteristics. Can we do this without GPS? Outline Steering
system: precise steering control
introduction steering system estimation vehicle control conclusion
Outline Steering system: precise steering control Conversion to
steer-by-wire System identification Steering control design Vehicle
control: infinitely variable handling characteristics Handling
modification Experimental results Estimation: steer-by-wire as an
observer Steering disturbance observer Vehicle state observer
Conclusion and future work introduction steering system estimation
vehicle control conclusion Sideslip estimation Yaw rate easily
measured, but sideslip angle much more difficultto measure
directly. Current approaches: GPS: loses signal under adverse
conditions optical ground sensor: very expensive Steer-by-wire
approach: Aligning moment transmits information about the
vehiclesmotionwe canceled it out, remember? Can be determined from
current applied to the steer-by-wireactuator. Steering system
dynamics
introduction steering system estimation vehicle control conclusion
Steering system dynamics road wheel angle moment of inertia damping
constant Coulomb friction aligning moment motor torque motor
constant motor current Steering system as a disturbance
observer
introduction steering system estimation vehicle control conclusion
Steering system as a disturbance observer Express in state space
form.Choose steering angle as output(measured state).Motor current
is input.Aligning moment isdisturbance to be estimated. Link
between aligning moment and sideslip angle
introduction steering system estimation vehicle control conclusion
Link between aligning moment and sideslip angle Aligning moment can
be expressed as function of the vehiclestates, and r, and the
input, d. Vehicle state observer
introduction steering system estimation vehicle control conclusion
Vehicle state observer Express in state space form.Steering angle
is input.Yaw rateand aligning moment (from the disturbance
observer) are outputs(measurements). Aligning moment and state
estimation
introduction steering system estimation vehicle control conclusion
Aligning moment and state estimation Choose disturbance observer
gain T so that A-TC is stable andxerr=x-xest approaches zero.
Estimated aligning moment
introduction steering system estimation vehicle control conclusion
Estimated aligning moment Not exact, but doesnt need to be.
data_012504b Estimated sideslip and yaw rate
introduction steering system estimation vehicle control conclusion
Estimated sideslip and yaw rate Sideslip estimate from observer is
comparable to estimate from GPS. data_012504b Experiment: normal
vs. reduced front cornering stiffness
introduction steering system estimation vehicle control conclusion
Experiment: normal vs. reduced front cornering stiffness State
feedback from observer: yaw results comparable to using GPS.
mo_041104_stetam3_a Experiment: normal vs. reduced front cornering
stiffness
introduction steering system estimation vehicle control conclusion
Experiment: normal vs. reduced front cornering stiffness Sideslip
results also comparable to using GPS. mo_041104_stetam3_a
introduction steering system estimation vehicle control conclusion
Conclusion Driving safety depends on a vehicles underlying
handlingcharacteristics. Can make handling characteristics anything
we want providedwe have: Precise active steering capability Full
knowledge of vehicle states Precise steering control requires
understanding of interactionbetween tire and road. Treated as
disturbance to be canceled out. Vehicle state estimation uses
interaction between tire and roadas source of information. Seen by
observer as force that govern vehicles motion. introduction
steering system estimation vehicle control conclusion Future work
Adaptive modeling to accommodate nonlinear handlingcharacteristics.
Apply knowledge of tire forces to determine where the limits areand
stay below them. Bounding uncertainty in observer-based sideslip
estimation. Apply control and estimation techniques to a dedicated
by-wirevehicle (Nissan project).